The invention concerns an MR tomograph, having a magnet system for generating a homogeneous magnetic field within a volume to be investigated and a gradient system for generating magnetic gradient fields which vary linearly in the volume to be investigated, wherein the magnet system and the gradient system permit access to the volume to be investigated along at least two axes, which intersect at an essentially right angle; a radio frequency (RF) transmitting and receiving system for irradiating RF pulses onto a measuring object to be investigated within the investigation volume and for receiving MR signals from the measuring object; and also an evaluation and display means for generating and displaying an MR image from the inside of the measuring object.
MR tomographs offering the possibility to insert the measuring object, in particular a patient, into the investigation volume, not only from an axial but also from a transverse direction are known e.g. from U.S. Pat. No. 5,168,211. Further developments of such MR tomographs offering the surgeon in charge sufficient transverse access to the investigation volume, which can be utilized e.g. for carrying out minimum-invasive operations with simultaneous monitoring of the operation via simultaneously generated MR slice images, are known e.g. from DE 40 29 477 A1, U.S. Pat. No. 5,414,360, U.S. Pat. No. 5,463,364 or U.S. Pat. No. 5,545,997, where various geometrical configurations of magnet systems and gradient systems are described which permit such transverse access to the investigation volume.
U.S. Pat. No. 5,570,073 describes a further possibility of almost lateral access to the investigation volume in addition to the axial access and shows an MR disk coil which is designed particularly compact in the axial direction such that access to the investigation volume at a relatively large angle with respect to the axis of the configuration is possible. Thus, minimum-invasive surgical treatment with simultaneous recording of MR slice images becomes also possible.
A further feasible possibility for a tomography system with lateral access to the homogeneity volume is a so-called "inside-out" arrangement in which the homogeneity volume is arranged outside of the actual tomograph owing to the particular construction of the magnet system. Such an arrangement is described e.g. in DE 296 09 289.4 U.
With respect to the above-mentioned minimum-invasive operations which can be carried out e.g. by means of a biopsy needle, it is furthermore known to provide a computer tomography (CT) image or an MRI image in the background. Up to now, said tomogram or projected image has been obtained previously, i.e. before the patient (or the object to be investigated) is removed from the tomograph, and then the operation is carried out outside of the tomograph, in particular outside of the investigation volume. In this connection, freely movable biopsy needles, provided with marking points (see e.g. the leaflet "VectorVision" by the company BrainLAB, Heimstetten, September 1996) and also stationary positioning systems (e.g. magazine article by Hubner and Kuhnapfel in Computer assisted Radiologie, 751-756, 1996) are used, wherein the mounting of the biopsy needle is displaceable in a manner defined by two circles and the needle is insertable in a defined manner. The position of the freely movable needle is determined optically e.g. by means of reflecting, simultaneously moving spheres (see leaflet "VectorVision"). The position of the needle is faded into the image which has been recorded previously at another location. In this connection, it is of course necessary to make sure that the coordinate systems coincide as exactly as possible in each case. This is probably achieved by means of defined mechanical strokes. In this connection, it seems to be absolutely necessary that the investigation object is positioned precisely and remains in place. For this purpose further optical marking points are stuck onto the object, if necessary.
The known biopsy methods with a "free" needle have the disadvantage that, during biopsy, the needle remains steady only to such an extent as provided by the hand of the surgeon guiding the needle. It is difficult to use the known methods having optical marking points in the inside of a tomography magnet system for on-line examinations since owing to the still confined circumstances and predetermined bedding, there are some obstacles in the way which block off the light, let alone the patient herself/himself or the person guiding the manipulator. For this reason, the defined manually guided manipulators which are mounted e.g. on a stereo-tactic ring have been used up to now only outside of the tomograph, or outside of its investigation volume having a homogeneous magnetic field.
On the other hand, when the operation is carried out outside of the imaging apparatus, the position of an inner location which is the object of the operation (e.g. in the brain, kidney, mamma etc.) can be different from its position during imaging. It is possible that an organ of the patient was moved owing to the transport, a biopsy needle used for the operation may displace tissue, a blood vessel may have changed its spatial position owing to pulsation etc. The absolute position of the manipulator is then possibly shown shifted by a few millimeters with respect to the real position such that the operation may not have the desired success or might even be lethal for the patient.
In the above-mentioned article by Hubner and Kuhnapfel it is generally said that the apparatus described therein offers the possibility of simultaneous diagnosis and on-line three-dimensionally based guiding assistance during surgical operations, however use of an MR tomograph offering transverse or at least angular lateral access is not mentioned therein and the operation is not carried out manually by a surgeon either. They rather suggest a "biopsy robot" which is supposed to carry out biopsy fully automatically, and if possible also via remote control, within a conventional tomograph without lateral accessibility.
Irrespective of the fact that systems of this kind do not yet exist concretely and are not used in practice, they would have the considerable disadvantage that such a robot configuration would have to be equipped with some kind of a motor which would undoubtedly comprise ferromagnetic parts which would impair to a considerable extent the homogeneity of the magnetic field in the investigation volume containing the robot. This would eventually cause the quality of the images of the MR tomograms obtained to be too bad to be useful for exact positioning of the biopsy needle. This might also be the reason why the biopsy robot apparatus suggested in theory in the above-mentioned magazine article by Hubner and Kuhnapfel has not yet been realized in an MR tomograph.